Method and system for designing an aircraft

ABSTRACT

A method for designing an aircraft includes defining an initial catalog of requirements for at least one aircraft design. An optimization of the at least one aircraft design is carried out based on the catalog of requirements in terms of anticipated operating costs. A predefined total flight network is simulated with the at least one aircraft design and a total flight network efficiency is determined. It is then checked as to whether the determined total flight network efficiency constitutes an optimum. The catalog of requirements is adapted and an iteration is performed upon a determination that the determined total flight network efficiency does not constitute the optimum.

CROSS-REFERENCE TO PRIOR APPLICATION

Priority is claimed to European Patent Application No. EP 13194503.2,filed on Nov. 26, 2013, the entire disclosure of which is herebyincorporated by reference herein.

FIELD

The invention relates to a method and a system for designing anaircraft, in particular a passenger aircraft and a freight aircraft inthe civil sector.

BACKGROUND

The design and construction of a new aircraft are very complex andextremely costly processes. Therefore an aircraft manufacturer who plansa new aircraft model, attempts to determine—before the actual design anddevelopment phase starts—in particular what technical operational andeconomic characteristics the envisaged new aircraft model should have inorder to be successful commercially and therefore justify the highdevelopment costs.

Up until now it has been customary to do this by invitingrepresentatives of airlines, and therefore representatives of potentialcustomers, to what are referred to as customer focus group or airlineadvisory board events. At these events, the representatives of theairlines are asked to express requests regarding the configurationparameters of a new aircraft model. The configuration parametersenquired about include, for example, the desired payload (this relatesessentially to the size of the fuselage with respect to the maximumnumber of passengers plus freight to be carried, for example), the rangerequirement, take off and landing requirements, the desired cruisingaltitude and, if appropriate, further configuration parameters such as,for example, the desired cruising speed, maximum values for the wingspan, fuselage length and/or surface pressure per wheel of the landinggear.

The estimates regarding the configuration parameters which are issued bythe representatives of the airlines are subsequently compiled andevaluated statistically. The aircraft manufacturer will then defineconfiguration parameters determined on the basis of the statisticalevaluation and according to further questioning of experts, as aconfiguration point for the new aircraft model.

On the basis of the configuration point determined in this way, asynthesis method is then applied which permits an aircraft design whichsatisfies the configuration point to be iteratively optimized in termsof the costs. The corresponding synthesis method according to the priorart is, inter alia, described in “Synthesis of Subsonic Airplane design:An introduction to the preliminary design of subsonic general aviationand transport aircraft, with emphasis on layout, aerodynamic design,propulsion and performance” by Egbert Torenbeek, published 1982 by DelftUniversity Press, 9th Reprint 1999, and in “Advanced Aircraft Design:Conceptual Design, Technology and Optimization of Subsonic CivilAirplanes” by Egbert Torenbeek, published 2013 by John Wiley & Sons.

In the synthesis method, firstly dimensioning of the propulsion system,wings and empennage, mass estimation resulting therefrom together withthe parameters of the configuration point and preliminary estimation offlight performance are optimized iteratively in such a way that themaximum takeoff weight converges and the range requirements and take offand landing requirements are satisfied according to the configurationpoint. Subsequently, a cost estimation is made for a correspondinglyoptimized design, wherein, in particular, the later operating costs areestimated. It is then checked whether the design is that which isoptimum in terms of costs. If this is not the case, the iterationdescribed above is frequently repeated with other output variables (forexample the dimensioning of the propulsion system) until cost optimum isachieved.

The method described gives rise to an aircraft design which is optimizedin terms of the predefined configuration point. However, in the methodsknown from the prior art it is not ensured that the configuration pointto which the aircraft design is optimized is also selected in an optimumway. For example, in the prior art the ultimate configuration point isdefined on the basis of customer consultations and subjective expertopinions, which undeniably entails the risk of selecting an “incorrectconfiguration point”. As a result, an aircraft can be dimensioned“beyond market requirements” and will then only find few customers, ifany at all. The consequences can be “fatal” owing to the highdevelopment costs for the aircraft manufacturer.

SUMMARY

In an embodiment, the present invention provides a method for designingan aircraft. In a step (a), an initial catalog of requirements isdefined for at least one aircraft design. In a step (b), an optimizationof the at least one aircraft design is carried out based on the catalogof requirements in terms of anticipated operating costs. In a step (c),a predefined total flight network is simulated with the at least oneaircraft design and a total flight network efficiency is determined. Itis then checked in a step (d) as to whether the determined total flightnetwork efficiency constitutes an optimum. In a step (e), the catalog ofrequirements is adapted and an iteration is performed starting form thestep (b) upon a determination that the determined total flight networkefficiency does not constitute the optimum.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in even greater detail belowbased on the exemplary figures. The invention is not limited to theexemplary embodiments. All features described and/or illustrated hereincan be used alone or combined in different combinations in embodimentsof the invention. The features and advantages of various embodiments ofthe present invention will become apparent by reading the followingdetailed description with reference to the attached drawings whichillustrate the following:

FIG. 1 shows a system according to an embodiment of the invention whichis designed to carry out the method according to an embodiment of theinvention; and

FIG. 2 shows a one-piece flowchart of the method according to anembodiment of the invention in the way in which it runs on the systemaccording to FIG. 1.

DETAILED DESCRIPTION

In an embodiment the present invention provides an improved method andsystem for designing an aircraft, which no longer have the disadvantagesfrom the prior art, or only have these disadvantages to a reduceddegree.

In one embodiment, the invention provides a method for designing anaircraft, comprising the steps:

-   -   a. defining an initial catalog of requirements for at least one        aircraft design;    -   b. carrying out optimization of the at least one aircraft design        on the basis of the catalog of requirements in terms of the        anticipated operating costs;    -   c. simulating a predefined total flight network with the at        least one aircraft design and determining the total flight        network efficiency;    -   d. checking whether the determined total flight network        efficiency constitutes an optimum; if not:    -   e. adapting the catalog of requirements and performing iteration        starting from step (b).

In another embodiment, the invention provides a system comprising:

-   -   a catalog of requirements memory for storing a catalog of        requirements for at least one aircraft design;    -   a synthesis module for determining aircraft designs optimized in        terms of costs, on the basis of the catalog of requirements from        the catalog of requirements memory;    -   a simulation module for carrying out a simulation of a total        flight network with the aircraft designs determined by the        synthesis module (3) and for determining the total flight        network efficiency; and    -   an optimization module for checking whether the total flight        network efficiency determined by the simulation module is an        optimum, and for changing the catalog of requirements in the        catalog of requirements memory if no optimum of the total flight        network efficiency is present.

The method according to an embodiment of the invention for designing anaircraft is defined in that not only is an aircraft design optimized toa specific, predefined catalog of requirements but in addition thecatalog of requirements is also optimized in such a way that an ultimateaircraft design is matched as well as possible to a predefined totalflight network. In contrast to the prior art, not only is an individualaircraft design optimized in terms of costs on the basis of a predefinedconfiguration point, but it is also ensured that the configuration pointfor an aircraft design is optimized in respect of a predefined totalflight network.

Before embodiments of the invention and the advantages thereof aredescribed further, firstly a number of terms which are used in relationto the invention will be explained in more detail.

The “catalog of requirements” contains a compilation of individualtechnical requirements relating to the at least one aircraft design.These requirements may comprise, for example, the payload, the range andtake off and landing requirements, for example for airports at highaltitudes in hot regions (hot and high airports) or particularly shortrunways. Minimum cruising altitude, minimum cruising speed and/ormaximum values for the wing span, fuselage length and/or surfacepressure per wheel of the landing gear may also be predefined.Alternatively or additionally to the minimum payload, requirementsrelating to the number of passenger seats may also be provided.

The catalog of requirements can comprise the technical requirements foran individual aircraft design. However, it is also possible for thecatalog of requirements to comprise corresponding requirements for aplurality of different aircraft designs. It is therefore possible, forexample, for a catalog of requirements to contain, on the one hand,requirements for a short-haul aircraft and, on the other hand,requirements for a long-haul aircraft. Certain links between therequirements for a plurality of different aircraft designs can also bepredefined in the catalog of requirements. It is therefore possible, forexample, to define that two aircraft designs are to have identical wingsin order to reduce the overall development costs for both aircraftdesigns.

A “total flight network” is the collection of all the flight routes ofone or more airlines which are served or are to be served by theseairlines. The total flight network is therefore not only limited tothose flight routes which it is to be possible to serve by a newaircraft design according to expert opinion. It is also possible for thetotal flight network which forms the basis for the method or systemaccording to embodiments of the invention to constitute merely a partialnetwork of a flight network which is actually present. For example, inorder to simplify the simulation of the total flight network individualroutes for which an already existing aircraft type has been specificallydeveloped (for example extreme long-haul) can be excluded from thesimulated total flight network. However, in this case also the totalflight network is still not limited only to those routes which it is tobe possible to serve by a new aircraft design according to expertopinion.

For the simulation of the total flight network, not only informationabout the length of the respective flight route but preferably alsoinformation about the take off and landing conditions at the individualairports of the respective flight route are available for eachindividual flight route. Furthermore, for example, maximum values forthe wing span, fuselage length and/or surface pressure of a wheel of thelanding gear can be predefined for the individual airports in the totalflight network. Furthermore, information about the payload volume on aflight route and/or information about the number of time windows atwhich a specific airline is allowed to start and land at an airport(airport slots) can also be available. Alternatively or additionally tothe payload volume, the passenger volume on a flight route can also beavailable.

The total flight network which is used as the basis for the simulationof the method according to an embodiment of the invention can be anactual total flight network of one or more airlines. However, it is alsopossible for a theoretical total flight network to be used as the basisfor the simulation. It is therefore possible, for example, to project afuture total flight network, which constitutes, for example, theanticipated or planned total flight network in the near future or in tenor twenty years, on the basis of an actual total flight network.

In the method according to an embodiment of the invention, firstly aninitial catalog of requirements for at least one new aircraft design isdefined. This initial catalog of requirements can contain the technicalrequirements for just one aircraft design. However, it is also possiblefor the initial catalog of requirements to contain technicalrequirements for a plurality of aircraft designs, for example one ormore short-haul aircraft and one or more long-haul aircraft.

Subsequently, optimization of the respective aircraft design is carriedout individually for each aircraft design for which the catalog ofrequirements contains technical requirements. The optimization iscarried out here primarily in terms of the operating costs of anaircraft design, wherein the operating costs can be calculated asfunctions of technical variables or of the configuration of the aircraftdesign. It is therefore possible, for example, to calculate the fuelconsumption and the associated costs from technical parameters such as,inter alia, the thrust/mass ratio and/or the aerodynamic quality of theaircraft design; the maintenance costs can be determined as a functionof the aircraft mass or the number of engines; airport taxes aredependent on the aircraft mass etc. Corresponding calculations of theanticipated costs for an aircraft design are known from the prior art.

In order to optimize the operating costs or the underlying technicalvariables it is possible to have recourse to the synthesis methodalready known from the prior art. In this method, firstly various designvariables such as dimensioning of the propulsion system, wings andempennage, and on the basis thereof at least to a certain extentresultant variables such as the lift distribution of the wing, mass,center of gravity, drag and flight performance are iterated until themaximum takeoff weight converges, while complying with the otherconstraints such as the range requirement, take off and landingrequirements etc. Subsequently, an estimate of the operating costs iscarried out and the iterative method described above is repeated untilan operating cost optimum is obtained. Anticipated operating costs canbe determined on the basis of technical variables of the aircraftdesign, as explained above.

With the aircraft design or designs optimized in this way, the totalflight network is then simulated and the total flight network efficiencyis determined. In this context, a flight route efficiency is determinedfor each flight route of the total flight network and the individualflight route efficiencies are summed to form the total flight networkefficiency. A flight route efficiency is calculated from the receiptswhich are anticipated for a specific aircraft design on a specificflight route, minus the costs for the use of the aircraft design on thisroute. Flight route efficiency values for a specific route can bedetermined here only for aircraft designs which meet the technicalrequirements for serving the flight route, for example that is to sayhave a sufficient range.

During the simulation of the total flight network it is possible for twoor more aircraft designs to be suitable for use on one specific flightroute. In this case, the efficiency potential of each individualaircraft design which is suitable for use on the respective flight routeis preferably calculated. Subsequently, the aircraft design whoseefficiency value contributes to the maximum total flight networkefficiency value during this simulation step is selected for this flightroute.

It is also possible that during the simulation individual flight routescannot be served by any of the simulated aircraft designs. In this case,the route which cannot be served can be associated with high costs,which ultimately results in a situation in which the total flightnetwork efficiency of a simulation during which the individual flightroutes cannot be served cannot constitute an optimum. Alternatively, thesimulation of the total flight network can also be discontinued if aflight route cannot be served by any of the simulated aircraft designs.

Constraints relating to the selection of the aircraft designs which areprovided for the individual flight routes can be predefined for thesimulation of the total flight network and the determination of thetotal flight network efficiency. It could therefore be possible, forexample, to predefine that in the total flight network or in a specificpart of the total flight network only a maximum number of differentaircraft designs are to be used and/or that any aircraft design used inthe total flight network is to be or must be capable of being used on aminimum number of individual flight routes in the total flight network.The maximum number of different aircraft designs is preferably greaterthan or equal to two here. Through corresponding constraints, a fleet ofdifferent aircraft designs on the total flight network, which is tooheterogeneous and therefore cannot be operated economically can alreadybe ruled out in this method step.

The anticipated receipts which are necessary for the calculation of aflight route efficiency can be calculated, for example, from the payloadvolume or passenger volume on a flight route, the available airportslots and the payload of an aircraft, which can also be converted intonumber of seats in the aircraft. In this context, empirical values suchas average loads and seasonal fluctuations in the passenger volume orpayload volume on a flight route can also be taken into account.

If the simulated flight route is a passenger route it is preferred ifthe passenger volume on the flight route is broken down into standardfare passengers and special fare passengers. Standard fare passengersare those who (have to) travel on a flight route for prescribed externalreasons (for example inflexible deadlines) and are therefore prepared tobuy a flight ticket even at the normal price. The group of standard farepassengers includes, in particular, business travelers. The special farepassengers are passengers who are not necessarily tied to one flightroute but are also prepared to select a different itinerary (for examplea detour) to arrive at their destination. Passengers of this type tendto use a dedicated flight route if they can buy inexpensive flighttickets. As a rule, such price-sensitive passengers often travel forpersonal reasons, such as for example holidays.

If the passenger volume is broken down into standard fare passengers andspecial fare passengers, a detailed calculation of the anticipatedreceipt is possible. In particular, during the simulation it istherefore possible to model the effect that in the case of aproportional change in the passenger volume of standard fare passengersin relation to special fare passengers, the average ticket price of thetickets on a flight route, and therefore also the flight routeefficiency changes. If, for example, the passenger volume of standardfare passengers increases and the passenger volume of special farepassengers remains unchanged, the portion of special fare passengers andtherefore the average ticket price of the tickets on a flight route andtherefore also the flight route efficiency drops.

Of course, a comparable situation also applies for various payloadswhich can also be broken down into a “standard rate” payload and a“special rate” payload. A standard rate payload can therefore relate topayloads which have to be transported quickly at a specific point intime, for example urgent spare parts, while special rate payloads canreadily also be transported by other means of transportation withoutwasting time if the other means of transportation are morecost-effective.

The costs which are also necessary to calculate a flight routeefficiency are the operating costs which are anticipated for a specificaircraft on the flight route. These anticipated operating costs can bedetermined or estimated in a known fashion by means of technicalvariables of the aircraft design. Operating costs can comprise fuelcosts, maintenance costs, personnel costs, airport charges etc. It isalso possible for the costs for a flight route to compriseproportionally the development costs for a new aircraft. The developmentcosts can be transferred portionally here, for example, to those flightroutes on which the newly developed aircraft is to be used according tothe simulation of the total flight network.

If the flight route efficiency is calculated for each flight route ofthe total flight network and summed to form the total flight networkefficiency, it is subsequently checked whether the total flight networkefficiency which is determined by means of the simulation is an optimum.If this is the case, an optimized aircraft design or a plurality ofoptimized aircraft designs are available. If an optimum has not yet beenreached, the catalog of requirements is changed and the described methodis run through iteratively until an optimum is reached.

Since the method according to an embodiment of the invention is aniterative method, checking as to whether an optimum is present will as arule not be possible at the first pass. In this case, the checking foran optimum has a negative result and the individual method steps arecorrespondingly iterated with a changed catalog of requirements untilthe checking for an optimum can actually be carried out and the presenceof an optimum becomes apparent. In this context it is basically possiblethat a total flight network efficiency which was determined during thefirst iteration steps proves to be an optimum, but at the point in timeof the determination thereof it was not possible to identify it as such.

A minimum number of iteration steps can also be predefined, wherein theiteration steps are respectively carried out with a changed catalog ofrequirements. If a number of total flight network efficiencies whichcorresponds to the number of minimum iterations is available on thebasis of different catalog of requirements, checking as to whether afurther total flight network efficiency which is calculated according tothe method according to an embodiment of the invention constitutes anoptimum is basically possible. However, iteration is also carried outhere until an actual optimum has been found.

Depending on the object which is set, the method according to anembodiment of the invention can determine a local or absolute optimum inthe total flight network efficiency. A local optimum can be desired, forexample, if a specific aircraft model which is already present is to bereplaced and a limited number of aircraft designs which are optimizedaccording to an embodiment of the invention is to close the resultinggap. An absolute optimum can be desired, in particular, when amultiplicity of different aircraft designs which are optimized accordingto an embodiment of the invention is to cover the widest possiblespectrum of requirements.

In addition the at least one optimized aircraft design, the result ofthe simulation of the total flight network can also be considered to bea further result of the method. From the result of the simulation it ispossible to infer which aircraft design should be used on which flightroute of the total flight network in order to achieve an optimum totalflight network efficiency.

In contrast to the prior art, in the method according to an embodimentof the invention an aircraft design is therefore not optimized solely inrespect of a predefined configuration point but instead thisconfiguration point is also optimized in the form of a variable catalogof requirements. As a result, one or more aircraft designs which aretherefore tailored to a predefined total flight network in such a waythat an optimum total flight network efficiency can be achieved areobtained.

During the adaptation of the catalog of requirements it is possible thatthe number of aircraft designs for which the catalog of requirementscontains requirements changes. It is therefore possible to determine inthe method an embodiment of according to the invention, for example,that the total flight network efficiency can be increased if instead ofan individual new aircraft design two new aircraft designs are providedinstead of the development costs which are increased as a result,wherein one of the two aircraft designs is optimized for a first part ofthe routes of the total flight network, while the other aircraft designis optimized for a second part of the routes of the total flightnetwork.

In the method according to an embodiment of the invention, boundaryconditions relating to the maximum number of possible aircraft designscan also be predefined. It is therefore possible, for example, to limitthe number of new aircraft designs or the level of the summeddevelopment costs for all the new aircraft designs. This permits thedevelopment capacities of aircraft manufacturers to be taken intoaccount.

In particular in the cases in which the number of new aircraft designsis limited, but also in all other cases, it is preferred if during thesimulation of the total flight network not only the aircraft designs onthe basis of the catalog of requirements but also aircraft models whichare already available are taken into account. In this context, all theaircraft models which are already available or only a number thereof canbe taken into account. The simulation of the total flight network thentherefore no longer remains limited to the new aircraft designs whichare determined with the synthesis method according to the catalog ofrequirements but rather also takes into account aircraft models whichare already available commercially. As a result it is as a rule possibleto serve all the flight routes of a flight network and to adapt the newaircraft design or designs in an optimum way to some of the routes inthe flight network, with the result that an optimum of the total flightnetwork efficiency is achieved by taking into account already availableaircraft models.

If the method according to an embodiment of the invention is to beprimarily used to replace a specific aircraft model which is alreadyavailable by a new aircraft design, the simulation can be carried outduring the method with the aircraft design of the other availableaircraft models. The specific available aircraft model is thenpreferably excluded from the simulation and the aircraft design isoptimized in such a way that it replaces the specific available aircraftmodel in the best way possible. The initial catalog of requirements forthe new aircraft design can be oriented here with respect to thetechnical characteristics of the specific aircraft model, but isoptimized in the course of the method according to an embodiment of theinvention as described.

The method according to an embodiment of the invention taking intoaccount already known aircraft models in the simulation will becomeclear with reference to an example. If, for example, a long-haulaircraft is available and if said aircraft is taken into account in thesimulation, the method according to an embodiment of the invention will,in the case of a total flight network comprising long-haul andshort-haul routes and in the case of the stipulation to optimize just asingle aircraft design, tend to give rise to an aircraft design which isadapted to the short-haul routes. In this context, the method accordingto an embodiment of the invention will automatically determine whichroutes are to be served with which aircraft, in order to achieve anoptimum total flight network efficiency. It is therefore conceivable,for example, that the total flight network efficiency is at a maximum ifthe majority of the short-haul routes are covered by the new aircraftdesign, but a small number of the short-haul connections with particulartake off and landing requirements are nevertheless served by the alreadyavailable long-haul aircraft. The method according to an embodiment ofthe invention namely takes into account the fact that if the newaircraft design were also to be suitable for the specified small numberof short-haul routes, the operating costs of the aircraft design wouldbasically increase in such a way that the efficiency on the othershort-haul routes would drop. In total, the total flight networkefficiency could therefore no longer be at a maximum. In contrast to thedevelopment methods according to the prior art, the method according toan embodiment of the invention in the specified example can thereforegive rise to an aircraft design for a short-haul aircraft which,although it cannot serve all the short-haul routes of the total flightnetwork, is advantageous overall even when specific short-haul routesare served by the already present long-haul aircraft.

In summary it is to be noted that the method according to an embodimentof the invention is distinguished by the fact that on the basis of acatalog of requirements with technical requirements for one or moreaircraft designs one or more optimized aircraft designs are determinediteratively taking into account variables which can be derived from thetechnical configuration of one or more aircraft designs, and a totalflight network. The aircraft designs which are obtained in this way aredistinguished by a configuration which is optimized with respect to thetotal flight network, as a result of which, on the one hand, the totalflight network efficiency can be increased, and, on the other hand, forexample, the total fuel consumption of the aircraft used on the totalflight network can also be reduced, since the aircraft which areprovided for the individual flight routes are adapted to these flightroutes as well as possible.

The system according to an embodiment of the invention is designed tocarry out the method according to an embodiment of the invention. For anexplanation of the system, reference is therefore made to the statementsabove.

It is preferred to provide a first database which is connected to thesimulation module and comprises information on all the flight routes ofthe total flight network. The simulation which is to be carried out bythe simulation module can then take place on the basis of the data fromthe first database.

It is further preferred to provide a second data base which is connectedto the simulation module and comprises information on available aircraftmodels. The simulation module is then preferably designed to take intoaccount this information while the simulation of the total flightnetwork is being carried out.

The system can also preferably be designed to carry out the methodaccording to an embodiment of the invention and the advantageousdevelopments of the method. For an explanation of these developments,reference is made to the statements above.

FIG. 1 illustrates a system 1 according to an embodiment of theinvention which is designed to carry out the method according to anembodiment of the invention. The system 1 comprises a catalog ofrequirements memory 2, a synthesis module 3, a simulation module 4 andan optimization module 5. The cited components are connected to oneanother in series. Furthermore, the optimization module 5 is alsoconnected to the catalog of requirements memory 2.

Furthermore, a first data base 6 and a second data base 7, are providedand are each connected to the simulation module 4. In the first database 6, a total flight network is modeled, i.e. information on all theflight routes of the total flight network is stored in the data base 6.Information on all the aircraft, or on some of the commerciallyavailable aircraft, is stored in the second data base 7.

A catalog of requirements comprising technical requirements for a newaircraft design is stored in the catalog of requirements memory 2. Thesetechnical requirements can comprise the minimum payload, the minimumrange and take off and landing requirements for one or more aircraftdesigns. In addition, requirements relating to the cruising altitudeand/or the cruising speed as well as maximum values for the wing span,fuselage length and/or surface pressure per wheel of the landing gearcan also be included. At the start, the catalog of requirements ispredefined manually, but, as explained below, it is changed iterativelyby the method 10 according to an embodiment of the invention (cf. FIG.2).

The catalog of requirements from the catalog of requirements memory 2 isfed to the synthesis module 3 which, on the basis of the technicalrequirements, carries out a synthesis method for optimizing one or moreaircraft designs in accordance with the technical requirements. Theresult of this synthesis method are one or more aircraft designs whichare based on the catalog of requirements and as far as possible haveoptimum and have the best possible, i.e. low, operating costs. These oneor more iteratively determined aircraft designs are transmitted to thesimulation module 4.

The simulation module 4 is designed to carry out a simulation of thetotal flight network which is modeled in the first data base 6. For thispurpose, the simulation module 4 calculates a flight route efficiencyfor each flight route of the total flight network, wherein the potentialefficiency is calculated on a flight route for all the availableaircraft designs which are transmitted by the synthesis module 3 and forthe available aircraft models which are stored in the second data base7, which are basically suitable in terms of their technicalspecifications for serving the flight route. The respective potentialefficiency which contributes to the highest total flight networkefficiency value in this simulation step is then selected as theultimate flight route efficiency.

Subsequently, the total flight network efficiency is formed as a sum ofthe individual flight route efficiencies by the simulation module 4 andis transmitted to the optimization module 5.

The optimization module 5 checks whether the total flight networkefficiency is an optimum. If this is the case, the aircraft design ordesigns which are determined by the synthesis module 3 are output asoptimized aircraft designs. If there is no optimum of the total flightnetwork efficiency available, the catalog of requirements in the catalogof requirements memory 2 is changed by the optimization module 5, andthe method is repeated iteratively until an optimum total flight networkefficiency is present.

The method which is carried out by the system 1 according to FIG. 1 willnow be explained in more detail with reference to the flowchart fromFIG. 2. Details such as how the individual components of the system 1according to FIG. 1 can be constructed can also be obtained from thefollowing statements.

At the start of the method 10 according to an embodiment of theinvention, an initial catalog of requirements is defined. Thisdefinition can be done by manual inputting (step 11). This catalog ofrequirements contains technical requirements for one or more aircraftdesigns. The technical requirements include minimum payload, minimumrange and take off and landing requirements. In addition, requirementsrelating to the cruising altitude and/or the cruising speed can also beincluded. The catalog of requirements can be directed to an individualaircraft design, that is to say contain merely one set of technicalrequirements. However, it is also possible for the requirements scheduleto contain requirements for a plurality of aircraft designs, wherein anumber of sets of technical requirements which corresponds to the numberof aircraft designs is then included.

The initial catalog of requirements is stored in such a way that, asexplained later, it can be changed during the method according to anembodiment of the invention (step 12).

On the basis of the catalog of requirements, firstly a synthesis method13 which is known from the prior art is carried out. In the synthesismethod 13, an aircraft design which is optimized in terms of theanticipated costs, in particular the anticipated operating costs, isdetermined for each aircraft design placed in the catalog ofrequirements.

In the synthesis method 13, firstly an initial fuselage dimensioning andan initial dimensioning of the propulsion system, wings and empennage iscarried out for an aircraft design. On the basis of these variables, anestimation of the mass can then be carried out and the lift distributionof the wings and the center of gravity can be determined, the drag ofthe aircraft design estimated and not least the flight performance ofthe aircraft design with the assumed dimensions can be determined (step14).

It is then checked whether the maximum takeoff weight of the aircraftdesign converges and the technical requirements from the catalog ofrequirements for this aircraft design, in particular range requirements,take off and landing requirements and, if appropriate, the cruisingaltitude, cruising speed and/or surface pressure per wheel of thelanding gear are satisfied (step 15). If this is not the case, thedimensioning of the fuselage and the dimensioning of the propulsionsystem, wings and empennage is changed and step 14 is carried out again.An iterative process occurs which is carried out until the maximumtakeoff weight of the aircraft design converges and the technicalrequirements from the catalog of requirements for this aircraft designare met.

If the corresponding checking in step 15 is positive, in a subsequentstep the anticipated costs for the aircraft design are calculated (step16). The operating costs can be determined here as a function oftechnical variables or the configuration of the aircraft design. It istherefore possible, for example, to calculate the fuel consumption andthe associated costs from the thrust/mass ratio of the aircraft design,and the maintenance costs can be determined as a function of theaircraft mass and the number of engines; airport taxes are dependent onthe aircraft mass etc.

In step 17 it is then checked whether an optimum is achieved in terms ofthe costs calculated in step 16. If this is not the case, thedimensioning of the fuselage and the dimensioning of the propulsionsystem, wings and empennage is changed again, and the process iscontinued with step 14. This is repeated until a cost optimum isreached. If this is the case, the aircraft design which is determined bymeans of this iterative method is fed to the following step 18.

The synthesis method 13 is carried out separately for each aircraftdesign for which technical requirements are present in the catalog ofrequirements, with the result that in the case of technical requirementsfor more than one aircraft design a correspondingly larger number ofaircraft designs are also fed to step 18.

In step 18, the total flight network is simulated. For this purpose, ineach case a flight route efficiency is determined for each route of thetotal flight network. For this purpose, the receipts and operating costswhich are anticipated for a flight route with a specific aircraft designor an aircraft model which is already available, such as can be stored,for example, in the first data base 6 (cf. FIG. 1), are offset againstone another. In addition to the actual operating costs, the developmentcosts of a new aircraft can also be included proportionally in thecalculation of the flight route efficiency. If more than one aircraftdesign or available aircraft is suitable, from the technical point ofview, for serving a specific flight route of the total flight network,the potential efficiency on the flight route is calculated for eachaircraft design or each available aircraft. The potential efficiencywhich contributes to the highest total flight network efficiency valuefor a particular route in this simulation step is selected as theultimate flight route efficiency for said flight route. During theselection of the aircraft designs or the available aircraft for theindividual flight routes in the total flight network, constraints suchas the maximum number of different aircraft types in the total networkor the minimum number of aircraft of a specific aircraft type can alsobe taken into account. Subsequently, the individual flight routeefficiencies are summed to form the total flight network efficiency.

In step 19 it is checked whether an optimum in terms of the total flightnetwork efficiency is present. If this is not the case, the catalog ofrequirements is changed and the method 10 starts again at step 12. Thecatalog of requirements is iterated here until an optimum of the totalflight network efficiency is present. In this context, the catalog ofrequirements can also be changed to such an extent that the number ofultimate aircraft designs changes.

If an optimum of the total flight network efficiency is reached, themethod 10 then ends with step 20 in which one or more optimized aircraftdesigns are then present. In addition to the at least one aircraftdesign, the result of the simulation from step 18 is then also present,from which result it is apparent which flight routes in the examinedtotal flight network should be served by the at least one aircraftdesign or other available aircraft in order to achieve an optimum totalflight network efficiency.

An embodiment of the invention will now be explained in more detail withreference to a numerical example. In this example, an aircraft familycomprising two aircraft which are suitable for long-haul routes is to bedesigned.

In the catalog of requirements, initially two sets of technicalrequirements, which each model the requirements of an aircraft design,are predefined here. For the first aircraft design the initialrequirements are with respect to the payload 60 t in the case of minimumrange of 6750 NM, while the second aircraft design is to initially havea payload of 50 t in the case of a minimum range of 7750 NM. The initialtake off and landing requirements for both aircraft designs are tailoredto the most unfavorable airport of the total flight network, i.e.according to the initial requirements both aircraft designs should beable to start and land at this airport with a runway length of 3000 mgiven an airfield reference temperature of 30° C. and an altitude 2500 mabove sea level (mean sea level). The initial requirements for the twoaircraft designs according to the catalog of requirements are combinedin the following table:

Payload Range Take off/landing conditions A 60 t 6750 NM 3000 m at 30°C. at 2500 m a.s.l. B 50 t 7750 NM 3000 m at 30° C. at 2500 m a.s.l.

In order to limit the development costs for the aircraft family, it isalso defined that both aircraft designs which are to be determined areto be equipped with identical wings and identical engines. Furthermore,it is defined that the result of the method according to an embodimentof the invention is to remain limited to a maximum of two aircraftdesigns.

In a first step, in a synthesis method for the two aircraft designs, ineach case the optimum configuration is determined in terms of theoperating costs of said designs. For this purpose, the various designvariables such as fuselage length and dimensioning of the propulsionsystem, wings and empennages is varied until in each case the maximumtakeoff weight converges, while the other constraints such as the rangerequirement and take off and landing requirements of the respectiveaircraft design are complied with. The maximum takeoff weight and therange characteristics or take off and landing characteristics of anaircraft design can be determined from the design variables andresulting intermediate variables such as the distribution of the lift ofthe wing, mass, center of gravity, drag and flight performance. In thiscontext it is to be noted that according to the specification in thepresent example the dimensioning of the propulsion system, wings andempennage is to be the same in both aircraft designs.

Subsequently, an estimate of the operating costs is made for each designand the prescribed iterative method is repeated until an operating costoptimum is present. The anticipated operating costs can be respectivelydetermined on the basis of the technical variables of the aircraftdesigns, as explained above.

As a result of the synthesis method, two aircraft designs are obtainedwhich have identical wings, empennages and engines, but differ in thefuselage length.

Subsequently, a simulation of the total flight network is carried out.In this example, the total flight network is to comprise a total of fiveflight routes:

Length Payload volume/day Aircraft slots/day 1 5500 NM 55 t 2 2 4500 NM130 t  3 3 7000 NM 40 t 2  4* 5000 NM 40 t 1 5 1200 NM 120 t  6

The route marked by “*” comprises the “most unfavorable” airport of thetotal flight network with a runway length of 3000 m at an airfieldreference temperature of 30° C. and an elevation 2500 m above sea level.The other routes each have take off and landing conditions which arecomparable to a runway length of 2500 m under standard conditions.

In addition to the two aircraft designs A and B, a known, commerciallyavailable aircraft model C is also taken into account in the simulation.The aircraft model C is a short-haul aircraft with the followingcharacteristics:

Payload Range Take off/landing conditions C 20 t 1500 NM 1600 m understandard conditions

In the simulation of the total flight network, the flight routeefficiency is then calculated for each individual flight route, wherein,firstly, in each case the potential efficiency is calculated on a routefor each aircraft design or each available aircraft model which canbasically be used on a flight route. For example, on flight routes 1, 2and 4 the aircraft designs A and B are basically used, while on theflight route 3 only the aircraft design B can be used, since theaircraft design A does not have the necessary maximum range. Theaircraft designs A and B as well as the already available aircraft modelC can be used on the flight route 5.

During the simulation it is determined what aircraft design or aircraftmodel makes it possible to achieve the maximum efficiency on a flightroute. For this purpose, the anticipated profits and the incurredoperating costs are determined for each route and offset against oneanother. A total flight network efficiency X is then obtained from thesum of the individual flight route efficiencies.

During the simulation of the total flight network on the basis of theaircraft designs A and B and the available aircraft model C, thefollowing breakdown can be obtained, for example:

Route Aircraft design/model Number flights/day 1 A 1 2 A 3 3 B 1 4 B 1 5C 6

Since at this point in time it is not possible to determine whether thetotal flight network efficiency X is an optimum, the method stepsdescribed above are carried out again, but the catalog of requirementsfor the aircraft designs is changed. The changed requirements for thetwo aircraft designs according to the catalog of requirements are:

Payload Range Take off/landing conditions A′ 65 t 5500 NM 2500 m understandard conditions B′ 40 t 7000 NM 3000 m at 30° C. at 2500 m a.s.l.

On the basis of the changed catalog of requirements for the aircraftdesigns A′ and B′, firstly operating-cost-optimized aircraft designs aredetermined by means of the synthesis method, which designs are then fed,together with the already known aircraft model C, to the simulation ofthe total flight network.

In the simulation of the total flight network on the basis of theaircraft designs A′ and B′ and the available aircraft model C, thefollowing breakdown can be obtained, for example:

Route Aircraft design/model Number flights/day 1 A 1 2 A 2 3 B 1 4 B 1 5C 6

The total flight network efficiency X′ which occurs during thissimulation is higher than the total flight network efficiency X from theprevious simulation. This is due to the fact that the aircraft design A′is now tailored well to the routes 1 and 2 and, for example, no longerhas to satisfy any particular take off and landing conditions, while theaircraft design B′ is tailored to the conditions of the routes 3 and 4.The already available (short-haul) flight model C continues to be wellsuited to serving the short-haul route 5.

The specified steps can be repeated with such a frequency that theoptimum of the total flight network efficiency is reached. In theillustrated exemplary embodiment it is to be assumed that the aircraftdesigns A′ and B′ are the aircraft designs which are optimum in terms ofthe total flight network to be examined. In addition to the informationabout the optimum aircraft designs A′ and B′, the method according to anembodiment of the invention also additionally provides the informationas to which aircraft design or which already available aircraft model isto be used on which flight routes in order to achieve the optimum totalflight network efficiency.

Of course, it is possible to refine the method according to theinvention which is illustrated only in one example. It is thereforepossible, for example, to base the simulation on a seasonal profile of apassenger volume instead of the payload volume per day, wherein thepassenger volume is preferably broken down into standard fare passengersand special fare passengers in order to be able to estimate better theanticipated efficiency on a flight route. It is, of course, alsopossible to take into account more flight routes and/or aircraft designsor aircraft models in the simulation. Complex relationships betweenindividual flight routes, for example as flight routes with anintermediate stop, can also be modeled. By means of the simulation it isalso possible to determine how many machines of an aircraft design oraircraft model are necessary to serve the examined total flight network.

The examined total flight network can be an actual current flightnetwork of one or more airlines. However, it is also possible to use anactual total flight network to project a future total flight networkwhich constitutes the anticipated total flight network in ten or twentyyears, for example. This provides the advantage that the at least oneaircraft design which is determined by means of the method according toan embodiment of the invention is adapted to the requirements toward theend of its development time.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive. Itwill be understood that changes and modifications may be made by thoseof ordinary skill within the scope of the following claims. Inparticular, the present invention covers further embodiments with anycombination of features from different embodiments described above andbelow. Additionally, statements made herein characterizing the inventionrefer to an embodiment of the invention and not necessarily allembodiments.

The terms used in the claims should be construed to have the broadestreasonable interpretation consistent with the foregoing description. Forexample, the use of the article “a” or “the” in introducing an elementshould not be interpreted as being exclusive of a plurality of elements.Likewise, the recitation of “or” should be interpreted as beinginclusive, such that the recitation of “A or B” is not exclusive of “Aand B,” unless it is clear from the context or the foregoing descriptionthat only one of A and B is intended. Further, the recitation of “atleast one of A, B and C” should be interpreted as one or more of a groupof elements consisting of A, B and C, and should not be interpreted asrequiring at least one of each of the listed elements A, B and C,regardless of whether A, B and C are related as categories or otherwise.Moreover, the recitation of “A, B and/or C” or “at least one of A, B orC” should be interpreted as including any singular entity from thelisted elements, e.g., A, any subset from the listed elements, e.g., Aand B, or the entire list of elements A, B and C.

The invention claimed is:
 1. A method for designing an aircraft, themethod comprising: a. defining an initial catalog of requirements for atleast one aircraft design, the catalog of requirements comprisingrequirements relating to one or more of a minimum payload, a minimumrange, take off and landing requirements, a minimum cruising altitude, aminimum cruising speed and maximum values for one or more of a wingspan, a fuselage length and a surface pressure per wheel of a landinggear for the at least one aircraft design; b. carrying out anoptimization of the at least one aircraft design based on the catalog ofrequirements in terms of anticipated operating costs; c. simulating apredefined total flight network with the at least one aircraft designand determining a total flight network efficiency; d. checking whetherthe determined total flight network efficiency constitutes an optimum;and e. adapting the catalog of requirements and performing an iterationstarting from step (b) upon a determination that the determined totalflight network efficiency does not constitute the optimum.
 2. The methodas claimed in claim 1, wherein the simulation in step (c) usesinformation about one or more of a length of a flight route, take offand landing conditions for any airport on individual flight routes andmaximum values for one or more of a wing span, a fuselage length and asurface pressure per wheel of the landing gear, which are made availablefor every flight route of the total flight network.
 3. The method asclaimed in claim 2, wherein the simulation in step (c) uses informationabout a payload volume or about airport slots at the airports on theindividual flight routes, which are made available for every flightroute of the total flight network.
 4. The method as claimed in claim 3,wherein the payload volume is used in the simulation in step (c) and isbroken down into a standard rate payload volume and a special ratepayload volume.
 5. The method as claimed in claim 1, wherein aircraftmodels which are already available are taken into account in thesimulation in step (c).
 6. The method as claimed in claim 1, wherein theadaptation of the catalog of requirements in step (e) gives rise to achange in a number of aircraft designs in the catalog of requirements.7. The method as claimed in claim 1, further comprising providingconstraints that are predefined in relation to a maximum number ofaircraft designs or maximum development costs for all of the aircraftdesigns.
 8. The method as claimed in claim 1, wherein the optimizationin step (b) is carried out by means of a synthesis method.
 9. A systemfor designing aircraft, the system comprising: a catalog of requirementsmemory storing a catalog of requirements for at least one aircraftdesign; a synthesis module configured to determine aircraft designsoptimized in terms of costs based on the catalog of requirements fromthe catalog of requirements memory; a simulation module configured tocarry out a simulation of a total flight network with the aircraftdesigns determined by the synthesis module and to determine a totalflight network efficiency; and an optimization module configured tocheck whether the total flight network efficiency determined by thesimulation module is an optimum and, upon a determination that thedetermined total flight network efficiency does not constitute theoptimum, to change the catalog of requirements in the catalog ofrequirements memory.
 10. The system as claimed in claim 9, furthercomprising a first data base connected to the simulation module andcontaining information on all flight routes of the total flight network.11. The system as claimed in claim 9, further comprising a second database connected to the simulation module and containing information onavailable aircraft models, the simulation module being configured totake into account the information on the available aircraft models whilethe simulation of the total flight network is being carried out.
 12. Thesystem as claimed in claim 9, wherein the system is configured to carryout a method for designing an aircraft, the method comprising: a.defining an initial catalog of requirements for the at least oneaircraft design; b. carrying out the optimization of the at least oneaircraft design based on the catalog of requirements in terms ofanticipated operating costs; c. simulating the total flight network withthe at least one aircraft design and determining the total flightnetwork efficiency; d. checking whether the determined total flightnetwork efficiency constitutes the optimum; and e. adapting the catalogof requirements and performing an iteration starting from step (b) upona determination that the determined total flight network efficiency doesnot constitute the optimum.
 13. The method as claimed in claim 1,further comprising manufacturing the aircraft in accordance with the atleast one aircraft design based on the determined flight networkefficiency constituting the optimum.
 14. A method for designing anaircraft, the method comprising: a. defining an initial catalog ofrequirements for at least one aircraft design, the catalog ofrequirements comprising requirements relating to one or more of aminimum payload, a minimum range, take off and landing requirements, aminimum cruising altitude, a minimum cruising speed and maximum valuesfor one or more of a wing span, a fuselage length and a surface pressureper wheel of a landing gear for the at least one aircraft design; b.carrying out an optimization of the at least one aircraft design basedon the catalog of requirements in terms of anticipated operating costs,the operating costs being calculated as functions of technical variablesor of a configuration of the at least one aircraft design; c. simulatinga predefined total flight network with the at least one aircraft designand determining a total flight network efficiency, the total flightnetwork efficiency being determined by summing up individual flightroute efficiencies, the flight route efficiencies each being calculatedusing receipts that are anticipated for use of the at least one aircraftdesign on a respective flight route minus costs for the use of the atleast one aircraft design on the respective flight route; d. checkingwhether the determined total flight network efficiency constitutes anoptimum; and e. adapting the catalog of requirements and performing aniteration starting from step (b) upon a determination that thedetermined total flight network efficiency does not constitute theoptimum.